Characterization Comparison Of Polycyclic Aromatic Hydrocarbon Uptake By Roots Of Different Crops

Polycyclic aromatic hydrocarbons (PAHs) have been of great concern because of their toxic, mutagenic, and carcinogenic properties, and recalcitrance for microorganisms. At present, soil and agricultural products have been heavily contaminated by PAHs. Therefore, the investigation regarding difference in PAH uptake by different crop roots and its mechanism is important since it can provide insights in understanding the mechanism of PAH transport into crop roots, and develop strategies for reducing PAH accumulation in agricultural products for safety, secreening the plant species for phytoremediation of PAH-contaminated soils or water. In the thesis, soybean (Glycine max), wheat (Triticum aestivum L.) and carrot (Daucus carotd) were employed as our research materials, and phenanthrene was selected as a model compound for PAHs in hydroponic experiments. The characterization comparison of phenanthrene (a representative of PAHs) uptake by different roots was investigated, the effects of PAH uptake on enzymatic activity were evaluated, electrophysiology experiments were performed to study the transmembrane process of PAHs, and the relationship between root morphological parameters, root components and PAH uptake was also discussed. The main results obtained are shown as follows:Soybean, carrot and wheat roots can take up phenanthrene from Hoagland nutrient solution and the phenanthrene absorbed by roots increases with incubation time. The uptake process consists of two sequential phases:a fast accumulation process followed by a slow one. The capability to take up phenanthrene for the three crops can be arranged as soybean>carrot>wheat. The relationship between the phenanthrene absorbed and time fits Elovich equation well. Uptake rate constants for soybean, carrot and wheat roots are4.31,4.10and2.84mg·(kg·h)-1, respectively. Concentration-dependent uptake of phenanthrene by roots of soybean, carrot and wheat can be described with Michaelis-Menten equation and the Km values for soybean, carrot and wheat are0.117,0.124and0.540mg·L-1. Hydroponic solution pH increases due to phenanthrene uptake and the trend in pH increase is in good line with those in uptake rate constant and Km value. In addition, the orders of uptake rate constant, Km value and pH increase for soybean, carrot and wheat are the same as that of uptake capability. Therefore, it is concluded that uptake rate constant, Km value and pH increase can be employed as indicators for capability to take up PAHs by crop roots, and uptake rate constant and Km value are better than pH increase as an indicator because of less affecting factors.The addition of phenanthrene in the uptake solution-triggered electrical response of soybean, wheat and carrot roots, consists of two sequential phases:depolarization followed by repolarization. The depolarization is phenanthrene concentration-dependent, with saturation kinetics. The depolarization of root cell membrane potential follows the order, i.e., soybean>carrot>wheat. The order for depolarization is good agreement with that for PHE uptake. Lower pH causes greater depolarization. For carrot, the depolarization pH5.5is similar to that at pH6.5. Vanadate and2,4-dinitrophenol (DNP) not only inhibit the PHE absorption of soybean and wheat, but also repress the depolarization.PHE can activate NR and plasma membrane H+-ATPase of soybean, carrot and wheat roots. The highest NR activity for wheat and carrot occurs at pH7.5, and for soybean at pH6.5. At pH7.0, H+-ATPase activity is highest for all the three crop roots. In the presence of PHE, the H+-ATPase activity of soybean roots is greater than the other two crop roots. The optimum pH for PEPase is7.0in the reaction solution, and PEPase activity decreases with increasing PHE concentrations. PHE inhibits the activation of PEPase by Mg2+. PPase activity of each corp reaches the maximum when the pH in reaction solution is7.5. The PPase activities of wheat and carrot have been strengthened by PHE. The PPase of soybean has no obvious response to PHE.The more the specific surface area and number of fibrous roots, the more phenanthrene is taken up by roots. The absorption of crop roots is also affected by the root components. Fat content of roots positively correlates with the PHE absorption. Carbohydrate also has a great contribution to the PHE absorption until it reaches a certain percentage in roots. Analysis of multiple linear regression indicates that specific surface area and fat content are the two key factors affecting root uptake of PHE, and the contribution of specific surface area to PHE uptake is greater than fat content.